1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image (hereinafter referred to as "toner") by an electrophotography process, electrographic recording method, or electrographic printing method or the like.
2. Background Art
In an electrophotography process, electrographic recording method, electrographic printing method, or the like, an electrostatic image on a photoconductive material is visualized by a toner. The toner is comprised mainly of a binder resin and a pigment. The visualized image is fixed directly on the photoconductive material or is transferred to other medium member and then fixed.
Therefore, characteristics required for the toner include, characteristics such as developing properties on the photoconductive material, transferring properties and fixing properties onto the paper. Moreover, a low energy fixing property can be required for energy conservation.
The heat-fixing process of the toner may be divided into a non-contact heating process such as an oven-fixing process, and a contact heating process such as a heated roller fixing process. The contact heating process may be desirable because it is highly heat efficient does not use large amount of electric power for heating fixing parts, and requires relatively compact.
However, the contact heating process has disadvantages in that it generates an offset phenomenon. Offset phenomenon is that parts of a toner forming an image transfers to the hot roller surface at a fixing time and is transferred to a subsequent paper to contaminate an image.
In order to prevent the generation of offset phenomena, for example, it is suggested to include compounds having releasability such as waxes, to the toner. However, when compounds such as waxes are contained in toners, the so-called "filming phenomena" (which is a phenomena in which a wax-like thin film adheres to the surface of the photoconductive material) occurs. Consequently, a toner comprising compounds having releasability has low storage stability.
It has also been suggested that a toner be comprised of polymers having high molecular weights. The generation of offset phenomena can thereby be prevented; however, another problem was then encountered. The softening temperature of the toner comprised of the polymer having high molecular weights was high, and consequently the fixing temperature became too high. Moreover, the polymer was tough, and therefore it is difficult to pulverize the polymer mixture in order to prepare toner having a uniform diameter.
The toner comprised of vinyl polymers having a broad molecular weight distribution, such as styrene as a binder resin, was suggested as the toner which can solve the above problems. The toner can be prevented from generating offset phenomena and has acceptable fixing properties at high temperatures. However, the fixing properties of the toner at low temperatures was worse.
Besides these toners, the toner comprised of polyester resin as a binder resin was suggested as a toner which can be fixed at a low fixing temperature. In the case of using a condensed resin such as polyester resin, polymers having a relative low molecular weight were used. However, these toners have problems of generating offset phenomena at high fixing temperatures.
Moreover, Japanese Patent Application, First Publication Sho 54-114245, Japanese Patent Application, First Publication Sho 58-1195, and Japanese Patent Application, First Publication Sho 58-14147 disclosed the toner comprised of vinyl polymers having high molecular weight resins and polyester resins having low molecular weight resins. These toners can prevent the generation of the offset phenomenon on caused by rising the temperature of the heat-fixing roller more than a constant temperature. However, mixing the polymers uniformly was difficult; therefore, it was difficult to prepare the toner having a uniform triboelectric properties.
Japanese Patent Application, First Publication Sho 54-86342, Japanese Patent Application, First Publication Sho 56-952, Japanese Patent Application, First Publication Sho 56-21136, Japanese Patent Application, First Publication Sho 56-168660, Japanese Patent Application, First Publication Sho 57-37353, Japanese Patent Application, First Publication Sho 58-14146, Japanese Patent Application, First Publication Sho 59-30542, Japanese Patent Application, First Publication Sho 61-105561, Japanese Patent Application, First Publication Sho 61-105563, Japanese Patent Application, First Publication Sho 61-124961, and Japanese Patent Application, First Publication Sho 61-275769 disclosed toners comprised of a resin having cross linking structures as monomer components of polyester resins, which is comprised of alcohols having three or more hydroxyl groups and/or carbonic acids having three or more carboxyl groups. However, when toners comprised of polyhydric alcohols and/or polycarboxylic acid at a ratio of 30 mol % or less per a toner, the cross linking reaction did not proceeded satisfactorily, therefore it was not possible to satisfactory offset preventing effects. In contrast, in the case of polyhydric alcohols and/or polycarboxylic acids at a ratio of 30 mol % or greater per toner, toners exhibited a good offset preventing property, but a moisture resistance of the toner was substantially deteriorated due to remaining non-reactional alcoholic hydroxyl groups or carboxyl groups in the carboxylic acids in the toner.
The object of the present invention is to provide a toner to solved the above problems and which has good developing properties, transferring properties, and fixing properties. In particular, the present invention provides a toner having good fixing properties at low temperatures, and which is suitable for a contact heated fixing process because it has offset preventing properties.